How can I make a 1970 El Camino safer? Should I us a 3 or 4 or 5 point harness or what?
How are you intending to drive it? Will it be a daily driver or a weekend racer? Has it been modified? What condition is the frame (structural rust)?
I will drive it around town but on interstates. Daily driver. No, but restored. good no rust.
I think a 3 point harness would do yet a 4 point has a tendency to push you down into the seat. It is safer but uncomfortable at times. 5 point, just not for daily use. All of this of course is my expeirience.
Could it stand in for the airbag since there is none?
If you are just looking to update a 1970’s era restraint system, you may want to consider a Schroth type set up:
http://www.schrothracing.com/products/Tuning.
A true racing harness that is properly installed and tightened will not allow you to…say reach the radio. An improper set-up can be worse than an old OEM.
I know of track cars that have failed state safety inspection for non DOT belts. 
The DOT has a different set of requirements than FIA or SFI
Ok, a 4 point will keep you from flying into the steering wheel better than a 3 point.
At what speeds do you plan to be driving at?
tops about maybe 80.
Go with the 4 point then. Those cars regardless of the restraint used, they always came apart on impact. I did not and do not like any care model of the 70’s era, just me of course.
Thanks everybody.
if you have any more ideas to make it any safer, please just post it please.
What belts are in there right now?
so other reading;
http://www.sparcousa.com/resourceFiles/16.pdf
http://www.saferacer.com/g-force-pro-4pt-2-inch-tuner-belts.html?productid=983
standard seatbelt
How about a roll cage and a helmet? 
(Puts on used car salesman hat)
I’ll trade you a newer vehicle with 3 point seat belts and airbags. I wouldn’t feel right letting you keep driving around in that old thing.
(Takes off UCS hat)
I think a bright paint job and a throaty exhaust would help.
A good 5 point harness, helmet, and roll cage will help you survive a crash.
Staying sober, obeying traffic laws, turning off the cell phone, paying attention to the traffic ahead will make your car safer.
Since the OP is planning on taking this vehicle up to…perhaps 80 mph, it would be a very good idea to upgrade the brakes. If they are, as I surmise, drum brakes, there are kits available for conversion to disks.
If the OP needs to make a panic stop from high speed, he may be shocked to learn that brake fade will take place on most drum brakes of that era during the course of one high speed panic stop, making the last few seconds of that stop an event that can be really scary. Hemmings Motor News would be a good source for this information.
yet more info…
http://www.soloracer.com/harnschrothfaq.html
I particularly like the Schroth Rallye 4 “Quick-Fit” belts that attach to normal seat belt receptacles.
Well what if it has no rear seat?